System and method for an accuracy-enhanced DLL during a measure initialization mode

ABSTRACT

A clock generator having a delay locked loop and a delay control circuit. The delay locked loop receives an input clock signal and adjusts an adjustable delay circuit to generate an output clock signal that is synchronized with received input clock signal. The delay control circuit coupled to the delay locked loop generates a control signal to initialize the delay measure operation to adjust the adjustable delay circuit, after comparing the phase difference of the input clock signal and the output clock signal. The delay control circuit further generates a start measure control signal to start measuring a delay applied to the measurement signal propagating through the adjustable delay circuit, and generates a stop measure control signal to stop the delay measurement of the measurement signal. The delay adjustment of the delay locked loop is then adjusted to apply the delay measurement when synchronizing the input and output clock signals.

TECHNICAL FIELD

This invention relates generally to integrated circuits, and more specifically to enhancing the accuracy of delay line based structures for synchronizing internal and external clock signals.

BACKGROUND OF THE INVENTION

In synchronous integrated circuits, the integrated circuit is clocked by an external clock signal and performs operations at predetermined times relative to the rising and falling edges of the applied clock signal. Examples of synchronous integrated circuits include synchronous memory devices such as synchronous dynamic random access memories (“SDRAMs”), synchronous static random access memories (“SSRAMs”), and packetized memories like SLDRAMs and RDRAMs, and include other types of integrated circuits as well, such as microprocessors. The timing of signals external to a synchronous memory device is determined by the external clock signal, and operations within the memory device are typically synchronized to external operations. For example, commands are placed on a command bus of the memory device in synchronism with the external clock signal, and the memory device must latch these commands at the proper times to successfully capture the commands. To latch the applied commands, an internal clock signal is developed in response to the external clock signal, and is typically applied to latches contained in the memory device to clock the commands into the latches. The internal clock signal and external clock must be synchronized to ensure the internal clock signal clocks the latches at the proper times to successfully capture the commands. In the present description, “external” refers to signals and operations outside of the memory device, and “internal” refers to signals and operations within the memory device. Moreover, although the present description is directed to synchronous memory devices, the principles described herein are equally applicable to other types of synchronous integrated circuits.

Internal circuitry in the memory device that generates the internal clock signal necessarily introduces some time delay, causing the internal clock signal to be phase shifted relative to the external clock signal. As long as the phase-shift is minimal, timing within the memory device can be easily synchronized to the external timing. However, with higher frequency clock signals, the time delay introduced by the internal circuitry becomes more significant. This is true because as the frequency of the external clock signal increases, the period of the signal decreases and thus even small delays introduced by the internal circuitry correspond to significant phase shifts between the internal and external clock signals. As a result of inherent delays, the commands applied to the memory device may no longer be valid by the time the internal clock signal clocks the latches.

To synchronize external and internal clock signals in modern synchronous memory devices, a number of different approaches have been considered and utilized, including delay locked loops (“DLLs”), as will be appreciated by those skilled in the art. As used herein, the term synchronized includes signals that are coincident and signals that have a desired delay relative to one another. FIG. 1 illustrates a conventional DLL circuit 100 for providing an approximate delay that closely matches the phase difference between input and output clock signals. The DLL circuit 100 uses a feedback configuration that operates to feed back a phase difference-related signal to control one or more delay lines, such as a coarse delay line 112 or a fine delay line 116, for advancing or delaying the timing of one clock signal to “lock” to a second clock signal.

An external clock signal is initially applied to the DLL circuit 100 and received by an input buffer 104 that provides a buffered clock signal DLY_REF to the DLL circuit 100. The DLY_REF signal is delayed relative to the external clock signal due to a propagation delay of the input buffer 104. The DLY_REF signal is then applied to coarse and fine delay lines 112, 116, which include a number of delay stages that are selected by a shift register 120 to apply a measured delay for adjusting the phase of the DLY_REF signal. The shift register 120 controls adjustments to the coarse and fine delay lines 112, 116 by providing shift control signals 134 in response to receiving control signals from a phase detector 130. In response to the shift control signals 134, the coarse delay line 112 applies a measured delay to adjust the phase of the DLY_REF signal near the desired phase for achieving the phase lock condition. The fine delay line 116 provides smaller delay adjustments to “fine tune” the DLY_REF signal closer to the desired phase lock condition. The coarse and fine delay lines 112, 116 generate an output signal CLK_OUT, whose phase is compared to the DLY_REF signal to determine whether the locking condition has been achieved. The CLK_OUT signal is provided to a model delay circuit 140 that duplicates inherent delays added to the applied external clock signal as it propagates through the delay loop, such as the input buffer 104 The model delay circuit 140 then provides a feedback signal DLY_FB to the phase detector 130. The phase detector 130 compares the phases of the DLY_REF signal and the DLY_FB signal to generate shift selection signals 132 to the shift register 120 to control the coarse or fine delay lines 112, 116. The shift selection signal instructs the shift register 120 to increase the delay of the coarse or fine delay lines 112, 116 when the DLY_FB signal leads the DLY_REF signal, or decrease the delay in the opposite case.

While the conventional DLL circuit 100 may adequately provide delay adjustments using the fine and coarse delay lines 112, 116, it is not without drawbacks. The DLL circuit 100 may take several cycles of the DLY_REF signal propagating through the coarse and fine delay lines 112, 116, making several attempts to properly adjust the DLY_REF signal before a locking condition is found, thereby further delaying the overall operation of the memory device. At higher clock frequencies, the DLL circuit 100 may not adequately control the coarse delay line 112, resulting in overcompensated adjustments. In such case, the DLL circuit 100 is required to restart the locking sequence, causing further delays and mismatched clock signals. For example, when the input and output clock signals are out of phase, the signal skew increases, resulting in faulty data transfers and undesired delays until the DLL circuit 100 locks again.

Other DLL circuits have been designed where the delay-locked loop is preset by a measurement function during an initialization mode. The phases of the internal and external clocks are placed in close proximity to each other by a predetermined delay adjustment during the initialization mode, thereby speeding up the locking time. However, DLL circuits that include initialization modes rely on additional circuitry, such as extra model delay circuits, switches and other logic components for measuring the predetermined delay. Inclusion of excess circuitry, adds further delays to the overall operation of the DLL circuit, utilizing more chip space and consuming more power. Furthermore, when model delay circuits are included for replicating delays within the delay loop, an exact delay match is rarely achieved due to design limitations of matched components as is known in the art. Mismatched delays generally result in the clock signals being out of phase and affect the DLL circuit's ability to achieve an accurate phase lock condition as described above.

Therefore, there is a need for a DLL circuit that more accurately achieves a faster locking state, and that reduces the use of excess circuitry, thereby reducing the overall size of the circuit and the consumption of power and space.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a conventional DLL circuit having a model delay circuit.

FIG. 2 is a block diagram of a DLL circuit according to an embodiment of the invention.

FIG. 3 is a timing diagram of various signals during operation of the DLL circuit according to an embodiment of the invention of FIG. 2.

FIG. 4 is a block diagram of a DLL circuit according to another embodiment of the invention.

FIG. 5 is a functional block diagram illustrating a memory device including a DLL circuit according to an embodiment of the invention.

FIG. 6 is a functional block diagram illustrating a computer system including the memory device of FIG. 5.

DETAILED DESCRIPTION

Embodiments of the present invention are directed to DLL circuits capable of enhancing the accuracy of delay line based structures by utilizing a measure initialization mode for synchronizing internal and external clock signals. Certain details are set forth below to provide a sufficient understanding of the invention. However, it will be clear to one skilled in the art that the invention may be practiced without these particular details. In other instances, well-known circuits, control signals, and timing protocols have not been shown in detail in order to avoid unnecessarily obscuring the invention.

FIG. 2 shows a block diagram of a DLL circuit 200 whose operation includes a measure initialization mode for initializing a measurement function for adjusting the phase difference between external and internal clock signals, thereby achieving a faster “phase locking state.” As in the DLL circuit 100, the DLL circuit 200 uses a feedback configuration that operates to feed back a phase difference-related signal for controlling one or more delay lines to advance or delay the timing of one clock signal until its rising edge is “locked” with the rising edge of a second clock signal. Similar to the DLL circuit 100, an external clock signal is applied to the DLL circuit 200 and received by an input buffer 204 that provides a buffered clock signal DLY_REF to the DLL circuit 200. The DLY_REF signal is delayed <-d1-> relative to the external clock signal due to a propagation delay of the input buffer 204.

The DLL circuit 200 uses a control signal MEAS_END to initialize and control when the system is in a measure initialization mode or when to transition into normal operation mode. After each reset of the system, the MEAS_END signal is initially set to LOW allowing a multiplexer 206 to couple the received DLY_REF signal to the coarse delay line 222, 223, and the system is placed in the measure initialization mode. The DLL circuit 200 may be reset by an external control logic block (not shown) or by an internal component such as a start counter circuit 234, which will be further described in later sections. With each reset of the DLL circuit 200, the coarse delay line 222, 223 is also reset to provide the smallest possible delay during the measure initialization phase, which may be the first unit delay cell of the coarse delay line 222, 223. The coarse delay line 222, 223 is set to a minimum delay at the start of the initialization mode to directly apply on the propagating clock signal any inherent delays, such as a delay <-d4-> from a hierarchical exit path 224, to initialize the measurement function. The multiplexer 206 receives the DLY_REF signal as the input to “0”, and the signal DLY_REF is propagated through the minimum delay setting. The multiplexer 206 generates an inverted input signal D_IN relative to the received DLY_REF, the relevancy of which will be further described in the next section. The D_IN signal is then provided by the coarse delay line 222, 223 through the hierarchical exit path 224, 226 and propagated through the fine delay line 227 with very minimal or no additional delays. The hierarchical exit path 224 applies an additional delay <-d4-> to the signal as it exits the delay line 222, 223 at a predetermined adjustment point. In previous designs, separate model delay circuits were used to apply the inherent delays, such as the delay <-d4-> of the hierarchical exit path 224. The D_N signal is then propagated through the output buffers 218 and 220 to generate a CLK_OUT signal.

Similar to the DLL circuit 100, a DLY_FB signal is generated in a feedback loop after the D_IN signal has propagated through the fine delay line 227 and the first output buffer 218, thereby returning the output signal to be sampled by the phase detector 230 to compare to the input signal phase. The DLY_FB signal is propagated through a model delay 232 that is designed to minimally apply a delay d1′+d3′ to compensate for the <-d1-> delay of the input buffer 204 and <-d3-> delay of the output buffer 220. The DLY_FB signal is then propagated directly to a first D flip-flop 236 to generate control signals for starting a delay measurement, which will be described next.

During the measure initialization mode, two D flip-flops 236, 238 are used to calculate a delay measurement that relies on the phase difference between the DLY_REF signal and the initialized DLY_FB signal, in order to obtain a faster locking state. A start counter circuit 234 receives the DLY_REF signal and counts several cycles at the beginning to allow the circuit to stabilize. Once the counts are complete to ensure a stable signal, the start counter circuit 234 transitions a LOW signal at the “D” input of the first D flip-flop 236 to HIGH and transitions the MEAS_END signal to HIGH to begin measuring a delay. Once the D-input transitions HIGH, the first D flip-flop 236 generates a measure pulse control signal MEAS_DLY (also HIGH) clocked by the received DLY_FB signal to start measuring the delay adjustment, and provides the MEAS_DLY signal to the “1” input of the multiplexer 206. The multiplexer 206 selects the received MEAS_DLY signal instead of the DLY_REF signal, and converts it to the inverted D_IN signal for propagating through the coarse delay line 222, 223 once again. The MEAS_DLY signal is also provided to a second D flip-flop 238 for generating a stop measuring control signal M_STROBE. The second D flip-flop 238 receives the MEAS_DLY signal as a HIGH signal to its “D” input from the output of the first D flip-flop 236. The second D flip-flop 238 samples the received HIGH MEAS_DLY signal to transition the M_STROBE signal HIGH, which is clocked by the DLY_REF signal. The M_STROBE signal is provided to a shift register 225, which then instructs the coarse delay line 222, 223 to stop measuring the delay adjustment. The HIGH M_STROBE signal is additionally provided to the start counter circuit 234, which transitions the MEAS_END signal LOW, and places the system in normal operation mode. During normal operation, the coarse delay line 222, 223 is set to provide the measured delay adjustment from the measure initialization operation. Therefore, a faster locking state is achieved for clock signals during normal operation by the preset coarse delay line 222, 223.

The system remains in normal operation until the DLL circuit 200 is once again reset by the start counter 234 or an external control logic block (not shown). During normal operation, the multiplexer 206 once again receives the DLY_REF signal as the input to “0”. The multiplexer 206 propagates the inverted D_IN signal through the coarse delay line 222, 223. The delay measurement calculated during the measure initialization mode is applied to provide the proper delay adjustment to the D_IN signal for “locking,” as previously described. After the adjusted delay is applied to the coarse delay line 222, 223, the adjusted signal is propagated through the hierarchical exit path 224 or 226 to be received by the fine delay line 227. The adjusted signal has now been optimally compensated for the <-d4-> delay due to direct use of the coarse delay line 222, 223 during the measure initialization operation.

Additionally, the DLL circuit 200 is designed with two possible pathway for the coarse and fine delay lines 222, 223, 227. If the calculated delay measurement requires an even number of unit delay stages, the D_IN signal passes through the coarse delay line 222 to the hierarchical exit path 224, which generates a CLKEVEN signal to the fine delay line 227. The fine delay line 227 is designed to provide an output signal having a phase variation from the input signal that is substantially smaller than the signal variation provided by the coarse delay lines 222, 223 to its output signal. If the delay measurement requires an odd number of unit delay stages, the D_IN signal is propagated through the coarse delay line 223 to the hierarchical exit path 226 instead. The hierarchical exit path 226 provides a CLKODD signal to the fine delay line 227. An embodiment of the fine and coarse delay lines 222, 223, 227 are described in detail in commonly assigned patent application Ser. No. 11/186,548 (filing date Jul. 21, 2005) entitled SEAMLESS COARSE AND FINE DELAY STRUCTURE FOR HIGH PERFORMANCE DLL, hereby incorporated by reference.

The adjusted output signal is then passed on to the output buffers 218, 220 to generate the output signal CLK_OUT. In addition, as previously described, the output signal is again returned to the phase detector 230 as the feedback signal DLY_FB after propagating through the buffer 218. During normal operation, the phase detector 230 compares the phases of the DLY_REF signal and the DLY_FB signal to provide a shift selection signal to the shift register 225. In response to the shift selection signal, the shift register 225 makes fine adjustments to the coarse delay lines 222, 223. The phase detector 230 may also be designed to provide a shift selection signal to the shift register (not shown) coupled to the fine delay lines 227. The shift selection signal instructs the shift register 225 to increase the delay of the coarse delay lines 222, 223 when the DLY_FB signal leads the DLY_REF signal, or decrease the delay in the opposite case. However, due to the preset adjustments of the measure initialization mode of operation of the DLL circuit 200, the phase of the DLY_REF and DLY_FB signals will be locked or close to locking at a faster rate, thereby reducing the need for any significant adjustments by the phase detector 230 and the shift register 225.

The calculation of the measure pulse MEAS_DLY will now be summarized. The sum of the delays allocated by the closed loop of the DLL circuit 200 from the DLY_REF signal to the DLY_FB signal N*tCK_(FB), where N is an integer and tCK is the clock period, can be represented by the equation N*tCK_(FB)=(measured delay)+d4+d2+d1′+d3′. Therefore, by rearranging the N*tCK_(FB) equation the measured delay is derived and represented by the equation measure delay=N*tCK_(FB)−(d4+d2+d1′+d3′). The total delay allocation of the delay loop in the DLL circuit 200 (applied to the external clock signal) can be represented as N*tCK_(REF)=d1+(measured delay)+d4+d2+d3. The two signals are “locked” when N*tCK_(FB)≈N*tCK_(REF) and is represented by an equation d1+[N*tCK_(FB)−(d4+d2+d1′+d3′)]+d4+d2+d3≈N*tCK_(REF). Therefore, the locking state is best achieved when the sum of delay elements (d4+d2+d1′+d3′) from the N*tCK_(FB) equation are accurately matched to the sum of delay elements (d4+d2+d1+d3) from the N*tCK_(REF) equation. The d1, d2, and d3 delays are the result of input or output buffers, which are easier to match. However, the d4 delay is an inherent delay due to the hierarchical exit path 224, 226. Such delays have previously been matched by incorporating a separate model circuit to the DLL circuit 200. However, separate model delays have difficulty achieving an exact delay match due to design limitations of matched components as previously described. According to the present invention, a better delay match is achieved if the original circuit path that provides the delay <-d4-> is used. By propagating the D_IN signal directly through the coarse delay line 222, 223 during the measure initialization mode (to initialize the measuring signal), the DLL circuit 200 applies the same <-d4-> delay as it would apply during normal operation. The accuracy of obtaining a locking point between the external and internal clock signals is significantly enhanced as shown by the equation d1+[N*tCK_(FB)−(d4+d2+d1′+d3′)]+d4+d2+d3≈N*tCK_(REF), if the delay <-d4-> for the N*tCK_(FB) and the N*tCK_(REF) are properly matched, thereby providing better matched signals where N*tCK_(FB)≈N*tCK_(REF).

Clock signals for the DLL circuit 200 during operation of the measure initialization mode are shown in FIG. 3. Each signal shown in FIG. 3 corresponds to signals labeled in FIG. 2. A sampling of the DLY_REF signal is shown, which has been delayed relative to the external clock signal due to the propagation delay of the input buffer 204 at time T0. The D_IN signal generated by the multiplexer 206 in response to receiving the DLY_REF signal is shown as being complementary to the DLY_REF signal at time T0, due to the inverting multiplexer 206. Also at time T0, the MEAS_END signal is shown having an initial value being LOW. After some delay (including d4, d2, and d1′+d3′), as the D_IN signal is propagated through the delay loop, the DLY_FB signal is generated at time T1 responsive to the rising edge of the DLY_REF signal at time T0. Also at time T1, the MEAS_END signal is transitioned HIGH by the start counter circuit 234, which initiates the delay measure command and further causes the multiplexer 206 to provide the MEAS_DLY signal as the D_IN signal. Shortly thereafter at time T2, the MEAS_DLY signal is transitioned HIGH in response to the rising edge of the DLY_Fb signal. The D_IN signal transitions LOW in response to the rising edge of the MEAS_DLY signal at time T3 to begin measuring the delay. After some measured delay, the second D flip-flop 238 generates a control signal M_STROBE that transitions HIGH at time T5 in response to the rising edge of the DLY_REF signal at time T4. The HIGH M_STROBE signal is received by the shift register 225 to control the coarse delay lines 222, 223 and is also received by the start counter circuit 234. In response, the shift registers stop measuring the delay and the MEAS_END signal transitions LOW at time T6 to initiate normal operation of the DLL circuit 200. A measured pulse is generated having a time width that is the same as the delay time between the rising edge of the MEAS_DLY signal at time T2 and the rising edge of the M_STROBE signal at time T5. At time T6, due to the MEAS_END signal, the multiplexer 206 switches to provide the DLY_REF signal as the D_IN signal. The DLY_FB signal responds to the changes in the D_IN signal waveform and after some delay between times T6 and T7, the DLY_FB signal follows the D_IN signal. The measured pulse in the D_IN signal also appears in the DLY_FB signal between times T8 and T9. As a result of the delay due to the measured pulse, the DLY_REF signal and the DLY_FB signal are locked, indicated by the rising edges of both signals matched at time T10.

An alternate embodiment of the invention is shown in FIG. 4, utilizing an SR flip-flop 442 in a DLL circuit 400. The DLL circuit 400 functions in essentially the same manner as the DLL circuit 200 of FIG. 2, and in the interest of brevity comparable components and functionality will not be described. As previously described, the DLL circuits 200, 400 may be reset by an external control logic block or by an internal component. In one embodiment, the DLL circuit 400 may include the SR flip-flop 442 to control the MEAS_END signal. The start counter circuit 234 may additionally provide an input signal to the S input of the SR flip-flop 442, to generate a HIGH output MEAS_END signal. The second D flip-flop 238 may additionally provide the M_STROBE signal to the R input of the SR flip-flop 442 to reset the MEAS_END signal to LOW after the locking delay is measured.

FIG. 5 is a functional block diagram of a memory device 500 including a clock generator 523 according to an embodiment of the invention. The memory device 500 in FIG. 5 is a double-data rate (“DDR”) SDRAM, although the principles described herein are applicable to any memory device that may include a delay-locked loop for synchronizing internal and external signals, such as conventional SDRAMs, as well as packetized memory devices like SLDRAMs and RDRAMs, and are equally applicable to any integrated circuit that must synchronize internal and external clocking signals.

The memory device 500 includes an address register 502 that receives row, column, and bank addresses over an address bus ADDR, with a memory controller (not shown) typically supplying the addresses. The address register 502 receives a row address and a bank address that are applied to a row address multiplexer 504 and bank control logic circuit 506, respectively. The row address multiplexer 504 applies either the row address received from the address register 502 or a refresh row address from a refresh counter 508 to a plurality of row address latch and decoders 510A-D. The bank control logic 506 activates the row address latch and decoder 510A-D corresponding to either the bank address received from the address register 502 or a refresh bank address from the refresh counter 508, and the activated row address latch and decoder latches and decodes the received row address. In response to the decoded row address, the activated row address latch and decoder 510A-D applies various signals to a corresponding memory bank 512A-D to thereby activate a row of memory cells corresponding to the decoded row address. Each memory bank 512A-D includes a memory-cell array having a plurality of memory cells arranged in rows and columns, and the data stored in the memory cells in the activated row is stored in sense amplifiers in the corresponding memory bank. The row address multiplexer 504 applies the refresh row address from the refresh counter 508 to the decoders 510A-D and the bank control logic circuit 506 uses the refresh bank address from the refresh counter when the memory device 500 operates in an auto-refresh or self-refresh mode of operation in response to an auto- or self-refresh command being applied to the memory device 500, as will be appreciated by those skilled in the art.

A column address is applied on the ADDR bus after the row and bank addresses, and the address register 502 applies the column address to a column address counter and latch 514 which, in turn, latches the column address and applies the latched column address to a plurality of column decoders 516A-D. The bank control logic 506 activates the column decoder 516A-D corresponding to the received bank address, and the activated column decoder decodes the applied column address. Depending on the operating mode of the memory device 500, the column address counter and latch 514 either directly applies the latched column address to the decoders 516A-D, or applies a sequence of column addresses to the decoders starting at the column address provided by the address register 502. In response to the column address from the counter and latch 514, the activated column decoder 516A-D applies decode and control signals to an I/O gating and data masking circuit 518 which, in turn, accesses memory cells corresponding to the decoded column address in the activated row of memory cells in the memory bank 512A-D being accessed.

During data read operations, data being read from the addressed memory cells is coupled through the I/O gating and data masking circuit 518 to a read latch 520. The I/O gating and data masking circuit 518 supplies N bits of data to the read latch 520, which then applies two N/2 bit words to a multiplexer 522. The circuit 518 provides 64 bits to the read latch 520 which, in turn, provides two 32 bits words to the multiplexer 522. A data driver 524 sequentially receives the N/2 bit words from the multiplexer 522 and also receives a data strobe signal DQS from a strobe signal generator 526 and a delayed clock signal CLK_OUT from the clock generator 523. The DQS signal is used by an external circuit such as a memory controller (not shown) in latching data from the memory device 500 during read operations. In response to the delayed clock signal CLK_OUT, the data driver 524 sequentially outputs the received N/2 bits words as a corresponding data word DQ, each data word being output in synchronism with rising or falling edges of the CLK signal that is applied to clock the memory device 500. The data driver 524 also outputs the data strobe signal DQS in synchronism with rising or falling edges of the CLK signal. Each data word DQ and the data strobe signal DQS collectively define a data bus DATA. As will be appreciated by those skilled in the art, the CLK_OUT signal from the DLL is a delayed version of the signal CLK signal, and the clock generator 523 adjusts the delay of the CLK_OUT signal relative to the signal CLK to ensure that the DQS signal and the DQ words are placed on the DATA bus in synchronism with the CLK signal, as previously described. The DATA bus also includes masking signals DM0-M, which will be described in more detail below with reference to data write operations.

During data write operations, an external circuit such as a memory controller (not shown) applies N/2 bit data words DQ, the strobe signal DQS, and corresponding data masking signals DM on the data bus DATA. A data receiver 528 receives each DQ word and the associated DM signals, and applies these signals to input registers 530 that are clocked by the DQS signal. In response to a rising edge of the DQS signal, the input registers 530 latch a first N/2 bit DQ word and the associated DM signals, and in response to a falling edge of the DQS signal the input registers latch the second N/2 bit DQ word and associated DM signals. The input register 530 provides the two latched N/2 bit DQ words as an N-bit word to a write FIFO and driver 532, which clocks the applied DQ word and DM signals into the write FIFO and driver in response to the DQS signal. The DQ word is clocked out of the write FIFO and driver 532 in response to the CLK signal, and is applied to the I/O gating and masking circuit 518. The I/O gating and masking circuit 518 transfers the DQ word to the addressed memory cells in the accessed bank 512A-D subject to the DM signals, which may be used to selectively mask bits or groups of bits in the DQ words (i.e., in the write data) being written to the addressed memory cells.

A control logic and command decoder 534 receives a plurality of command and clocking signals over a control bus CONT, typically from an external circuit such as a memory controller (not shown). The command signals include a chip select signal CS*, a write enable signal WE*, a column address strobe signal CAS*, and a row address strobe signal RAS*, while the clocking signals include a clock enable signal CKE* and complementary clock signals CLK, CLK*, with the “*” designating a signal as being active low. The command signals CS*, WE*, CAS*, and RAS* are driven to values corresponding to a particular command, such as a read, write, or auto-refresh command. In response to the clock signal CLK or the combination of clock signals CLK, CLK*, the command decoder 534 latches and decodes an applied command, and generates a sequence of clocking and control signals that control the components 502-532 to execute the function of the applied command. The clock enable signal CKE enables clocking of the command decoder 534 by the clock signal CLK or clock signals CLK, CLK*. The command decoder 534 latches command and address signals at edges of the CLK signal or CLK and CLK* signals (i.e., the crossing point of CLK going high and CLK* going low), while the input registers 530 and data drivers 524 transfer data into and from, respectively, the memory device 500 in response to the edges of the data strobe signal DQS and thus at the frequency of the clock signal CLK or clock signals CLK, CLK*. This is true because the DQS signal has the same frequency as the clock signals. The memory device 500 is referred to as a double-data-rate device because the data words DQ being transferred to and from the device are transferred at double the rate of a conventional SDRAM, which transfers data at a rate corresponding to the frequency of the applied clock signal. The detailed operation of the control logic and command decoder 534 in generating the control and timing signals is conventional, and thus, for the sake of brevity, will not be described in more detail.

FIG. 6 is a block diagram of a processor-based system 600 including processor circuitry 602, which includes the memory device 500 of FIG. 5. Typically, the processor circuitry 602 is coupled through address, data, and control buses to the memory device 500 to provide for writing data to and reading data from the memory device. The processor circuitry 602 includes circuitry for performing various processing functions, such as executing specific software to perform specific calculations or tasks. In addition, the processor-based system 600 includes one or more input devices 604, such as a keyboard or a mouse, coupled to the processor circuitry 602 to allow an operator to interface with the processor-based system 600. Typically, the processor-based system 600 also includes one or more output devices 606 coupled to the processor circuitry 602, such as output devices typically including a printer and a video terminal. One or more data storage devices 608 are also typically coupled to the processor circuitry 602 to store data or retrieve data from external storage media (not shown). Examples of typical storage devices 608 include hard and floppy disks, tape cassettes, compact disk read-only (“CD-ROMs”) and compact disk read-write (“CD-RW”) memories, and digital video disks (“DVDs”).

From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. For example, many of the components described above may be implemented using either digital or analog circuitry, or a combination of both. Accordingly, the invention is not limited except as by the appended claims. 

1. A clock generator comprising: a delay locked loop having at least one input to which an input clock signal is applied, and at least one output at which an output clock signal is provided, the delay locked loop operable to adjust the input clock signal by propagating it through an adjustable delay line to generate an output clock signal synchronized with the input clock signal; and a delay control circuit coupled to the respective inputs and outputs of the delay locked loop, the delay control circuit operable in a delay measurement mode to measure a delay by propagating a measurement signal through the adjustable delay line of the delay locked loop, the delay control circuit further operable upon entering a normal operation mode to stop the measurement of the delay and set the adjustable delay time to delay the input signal according to the measured delay.
 2. The clock generator of claim 1 further comprises a phase detector having a first input coupled to the input clock signal and a second input coupled to the output clock signal, the phase detector configured to determine the phase difference between the output clock signal and the input clock signal, and to provide fine adjustments to the adjustable delay line during normal operation mode of the delay locked loop.
 3. The clock generator of claim 2 wherein the delay locked loop further comprises a shift register coupled to the adjustable delay line, the shift register operable to receive a shift selection signal from the phase detector during normal operation mode and a shift selection signal from the delay control circuit during the delay measurement mode.
 4. The clock generator or claim 2 further comprising: a feedback loop coupled to the delay locked loop, the delay control circuit and the second input of the phase detector, the feedback loop operable to generate a feedback signal from the output clock signal of the delay locked loop to apply to the second input of the phase detector during normal operation mode of the delayed locked loop, and to provide the feedback signal to the delay control circuit for synchronizing delay measurement operations during the delay measurement mode.
 5. The clock generator of claim 4 wherein the delay control circuit further comprises: a first logic control circuit configured to receive the applied input clock signal and the feedback signal to generate a start control signal to start measuring the delay applied to the adjustable delay line of the delay locked loop; and a second logic control circuit coupled to the first control logic circuit, and configure to receive the start control signal and the applied input clock signal to generate a stop control signal to stop measuring the delay applied to the adjustable delay line of the delay locked loop.
 6. The clock generator of claim 5 wherein the first and second logic control circuits comprise a first and a second D flip-flop.
 7. The clock generator of claim 5 wherein the delay control circuit further comprises a start counter coupled to receive the applied input clock signal and the stop control signal, the start counter configured to count several cycles of the applied input clock signal and operable to provide a stabilized input clock signal to the first logic control circuit, the start counter further operable to provide an initialization control signal to initialize the delay measurement mode in response to receiving the input clock signal, and to reset the count in response to receiving the stop control signal.
 8. The clock generator of claim 5 wherein the delay control circuit further comprises a third logic control circuit coupled to receive the input clock signal and the stop control signal from the second logic control circuit, the third logic control signal operable to provide a reset control signal to the delay locked loop reset the delay measurement mode.
 9. The clock generator of claim 8 wherein the third logic control circuit comprises an SR flip-flop.
 10. The clock generator of claim 1 wherein the delay locked loop further comprises a coarse adjustable delay line having larger adjustable delays and a fine adjustable delay line having smaller adjustable delays.
 11. The clock generator of claim 10 wherein the adjustable delay line comprises a first coarse adjustable delay line for measuring an even number of unit delay cells and a second coarse adjustable delay line for measuring an odd number of unit delay cells.
 12. The clock generator of claim 1 wherein the adjustable delay line of the delay locked loop further comprises: a plurality of delay units, wherein a subset of delay units are selected according to the measurement signal of the delay control circuit; and an exit path circuit coupled to the plurality of delay units and operable to exit a clock signal propagating through the adjustable delay line at a delay unit selected by the measurement signal of the delay control circuit.
 13. A delay locked loop circuit comprising: an adjustable delay circuit having at least one input to which an input clock signal is applied and at least one output to which an output clock signal is provided, the adjustable delay circuit further having a coarse delay circuit having a plurality of delay units coupled to the at least one input, the adjustable delay circuit configured to adjust the coarse delay circuit relative to a measured delay to generate the output clock signal synchronized with the input clock signal; a hierarchical exit path coupled to the coarse adjustable delay circuit to propagate an adjusted output signal to the at least one output; a feedback loop coupled to the respective inputs and outputs of the adjustable delay circuit, the feedback loop configured to generate a feedback signal for comparing to a phase of the input signal and for controlling adjustments to the adjustable delay circuit; a measure control circuit coupled to the adjustable delay circuit and the feedback loop, the measure control circuit configured to provide a first control signal to the adjustable delay circuit responsive to the feedback signal and initialize a measure signal through a first unit delay of the coarse adjustable delay circuit, the measure control circuit further configured to generate a second control signal to begin measuring the measured delay by propagating the measure signal through subsequent unit delays of the coarse adjustable delay circuit and a third control signal to stop measuring the delay relative to the phase difference between the feedback signal and the input signal.
 14. The delay locked loop circuit of claim 13 further comprising: a phase detector coupled to receive the feedback signal of the feedback loop and the input clock signal applied to the adjustable delay circuit, the phase detector configured to determine the phase difference between the feedback signal and the input clock signal, and to control the adjustments to the adjustable delay circuit.
 15. The delay locked loop circuit of claim 14 wherein the measure control circuit further comprises: a first control logic circuit configured to receive the applied input clock signal and the feedback signal to generate the second control signal to start measuring the delay applied to the intermediate signal; and a second control logic circuit coupled to the first control logic circuit, configure to receive the second control signal and the applied input clock signal to generate the third control signal to stop measuring the delay applied to the intermediate signal.
 16. The delay locked loop circuit of claim 15 wherein the first and second control logic circuits comprises first and second D flip-flops.
 17. The delay locked loop circuit of claim 15 wherein the measure control circuit further comprises a start counter coupled to receive the applied input clock signal and the third control signal, the start counter configured to count several cycles of the applied input clock signal and provide a stabilized input clock signal to the first control logic circuit, and the start counter further configured to provide a start control signal to initiate the delay measure function responsive to the applied input clock signal and to provide a stop control signal to end the delay measure function responsive to the received third control signal.
 18. The delay locked loop circuit of claim 15 wherein the adjustable delay circuit further comprises a shift register coupled to the coarse delay circuit, the phase detector and the second control logic circuit, the shift register operable to provide a shift selection signal to the coarse delay circuit to shift the delay to either the right or the left responsive to control signals from the phase detector or second control logic circuit.
 19. The delay locked loop circuit of claim 13 wherein the adjustable delay circuit further comprises a fine adjustable delay circuit configured to adjust the phase of the output clock signal in smaller increments than the coarse adjustable delay circuit.
 20. The delay locked loop circuit of claim 13 wherein the coarse adjustable delay circuit of the adjustable delay circuit further comprises a first coarse delay line for measuring an even number of unit delay cells and a second coarse delay line for measuring an odd number of unit delay cells.
 21. The delay locked loop circuit of claim 13 further comprising a third logic control circuit configured to provide a reset control signal to reset the measure function of the delay locked loop circuit.
 22. The delay locked loop circuit of claim 21 wherein the third logic control circuit comprises an SR flip-flop.
 23. A memory device, comprising: an address bus; a control bus; a data bus; an address decoder coupled to the address bus; a read/write circuit coupled to the data bus; a control circuit coupled to the control bus; an array of memory cells coupled to the address decoder, control circuit, and read/write circuit; and a clock generator coupled to the control circuit, the clock generator comprising: a delay locked loop having at least one input to which an input clock signal is applied, and at least one output at which an output clock signal is provided, the delay locked loop operable to adjust the input clock signal by propagating it through an adjustable delay line to generate an output clock signal synchronized with the input clock signal; and a delay control circuit coupled to the respective inputs and outputs of the delay locked loop, the delay control circuit operable in a delay measurement mode to measure a delay by propagating a measurement signal through the adjustable delay line of the delay locked loop, the delay control circuit further operable upon entering a normal operation mode to stop the measurement of the delay and set the adjustable delay time to delay the input signal according to the measured delay.
 24. The memory device of claim 23 wherein the clock generator further comprises a phase detector having a first input coupled to the input clock signal and a second input coupled to the output clock signal, the phase detector configured to determine the phase difference between the output clock signal and the input clock signal, and to provide fine adjustments to the adjustable delay line during normal operation mode of the delay locked loop.
 25. The memory device of claim 24 wherein the delay locked loop further comprises a shift register coupled to the adjustable delay line, the shift register operable to receive a shift selection signal from the phase detector during normal operation mode and a shift selection signal from the delay control circuit during the delay measurement mode
 26. The memory device of claim 24 wherein the clock generator further comprises a feedback loop coupled to the delay locked loop, the delay control circuit and the second input of the phase detector, the feedback loop operable to generate a feedback signal from the output clock signal of the delay locked loop to apply to the second input of the phase detector during normal operation mode of the delayed locked loop, and to provide the feedback signal to the delay control circuit for synchronizing delay measurement operations during the delay measurement mode.
 27. The memory device of claim 26 wherein the delay control circuit further comprises: a first logic control circuit configured to receive the applied input clock signal and the feedback signal to generate a start control signal to start measuring the delay applied to the adjustable delay line of the delay locked loop; and a second logic control circuit coupled to the first control logic circuit, and configure to receive the start control signal and the applied input clock signal to generate a stop control signal to stop measuring the delay applied to the adjustable delay line of the delay locked loop.
 28. The memory device of claim 27 wherein the first and second logic control circuits comprise a first and a second D flip-flop.
 29. The memory device of claim 27 wherein the delay control circuit further comprises a start counter coupled to receive the applied input clock signal and the stop control signal, the start counter configured to count several cycles of the applied input clock signal and operable to provide a stabilized input clock signal to the first logic control circuit, the start counter further operable to provide an initialization control signal to initialize the delay measurement mode in response to receiving the input clock signal, and to reset the count in response to receiving the stop control signal.
 30. The memory device of claim 27 wherein the delay control circuit further comprises a third logic control circuit coupled to receive the input clock signal and the stop control signal from the second logic control circuit, the third logic control signal operable to provide a reset control signal to the delay locked loop reset the delay measurement mode.
 31. The memory device of claim 30 wherein the third logic control circuit comprises an SR flip-flop.
 32. The memory device of claim 23 wherein the delay locked loop further comprises a coarse adjustable delay line having larger adjustable delays and a fine adjustable delay line having smaller adjustable delays.
 33. The memory device of claim 32 wherein the adjustable delay line comprises a first coarse adjustable delay line for measuring an even number of unit delay cells and a second coarse adjustable delay line for measuring an odd number of unit delay cells.
 34. The memory device of claim 24 wherein the adjustable delay line of the delay locked loop further comprises: a plurality of delay units, wherein a subset of delay units are selected according to the measurement signal of the delay control circuit; and an exit path circuit coupled to the plurality of delay units and operable to exit a clock signal propagating through the adjustable delay line at a delay unit selected by the measurement signal of the delay control circuit.
 35. A computer system, comprising: a data input device; a data output device; a processor coupled to the data input and output devices; and a memory device coupled to the processor, comprising: an address bus; a control bus; a data bus; an address decoder coupled to the address bus; a read/write circuit coupled to the data bus; a control circuit coupled to the control bus; an array of memory cells coupled to the address decoder, control circuit, and read/write circuit; and a clock generator coupled to the control circuit, the clock generator comprising: a delay locked loop having at least one input to which an input clock signal is applied, and at least one output at which an output clock signal is provided, the delay locked loop operable to adjust the input clock signal by propagating it through an adjustable delay line to generate an output clock signal synchronized with the input clock signal; and a delay control circuit coupled to the respective inputs and outputs of the delay locked loop, the delay control circuit operable in a delay measurement mode to measure a delay by propagating a measurement signal through the adjustable delay line of the delay locked loop, the delay control circuit further operable upon entering a normal operation mode to stop the measurement of the delay and set the adjustable delay time to delay the input signal according to the measured delay.
 36. The computer system of claim 35 wherein the clock generator further comprises a phase detector having a first input coupled to the input clock signal and a second input coupled to the output clock signal, the phase detector configured to determine the phase difference between the output clock signal and the input clock signal, and to provide fine adjustments to the adjustable delay line during normal operation mode of the delay locked loop.
 37. The computer system of claim 36 wherein the delay locked loop further comprises a shift register coupled to the adjustable delay line, the shift register operable to receive a shift selection signal from the phase detector during normal operation mode and a shift selection signal from the delay control circuit during the delay measurement mode.
 38. The computer system of claim 36 wherein the clock generator further comprises a feedback loop coupled to the delay locked loop, the delay control circuit and the second input of the phase detector, the feedback loop operable to generate a feedback signal from the output clock signal of the delay locked loop to apply to the second input of the phase detector during normal operation mode of the delayed locked loop, and to provide the feedback signal to the delay control circuit for synchronizing delay measurement operations during the delay measurement mode.
 39. The computer system of claim 38 wherein the delay control circuit further comprises: a first logic control circuit configured to receive the applied input clock signal and the feedback signal to generate a start control signal to start measuring the delay applied to the adjustable delay line of the delay locked loop; and a second logic control circuit coupled to the first control logic circuit, and configure to receive the start control signal and the applied input clock signal to generate a stop control signal to stop measuring the delay applied to the adjustable delay line of the delay locked loop.
 40. The computer system of claim 39 wherein the first and second logic control circuits comprise a first and a second D flip-flop.
 41. The computer system of claim 39 wherein the delay control circuit further comprises a start counter coupled to receive the applied input clock signal and the stop control signal, the start counter configured to count several cycles of the applied input clock signal and operable to provide a stabilized input clock signal to the first logic control circuit, the start counter further operable to provide an initialization control signal to initialize the delay measurement mode in response to receiving the input clock signal, and to reset the count in response to receiving the stop control signal.
 42. The computer system of claim 39 wherein the delay control circuit further comprises a third logic control circuit coupled to receive the input clock signal and the stop control signal from the second logic control circuit, the third logic control signal operable to provide a reset control signal to the delay locked loop reset the delay measurement mode.
 43. The computer system of claim 42 wherein the third logic control circuit comprises an SR flip-flop.
 44. The computer system of claim 35 wherein the delay locked loop further comprises a coarse adjustable delay line having larger adjustable delays and a fine adjustable delay line having smaller adjustable delays.
 45. The computer system of claim 44 wherein the adjustable delay line comprises a first coarse adjustable delay line for measuring an even number of unit delay cells and a second coarse adjustable delay line for measuring an odd number of unit delay cells.
 46. The computer system of claim 35 wherein the adjustable delay line of the delay locked loop further comprises: a plurality of delay units, wherein a subset of delay units are selected according to the measurement signal of the delay control circuit; and an exit path circuit coupled to plurality of delay units and operable to exit a clock signal propagating through the adjustable delay line at a delay unit selected by the measurement signal of the delay control circuit.
 47. A method of generating a clock signal, comprising: receiving an input clock signal; initializing a measure signal having a minimum adjustable delay responsive to the input clock signal; determining a phase difference between the input clock signal and the measure signal; measuring a delay applied to the measure signal relative to the phase difference; measuring the delay applied to the measure signal relative to the phase difference; applying the measured delay of the measure signal to the received input clock signal to provide an output clock signal synchronized with the input clock signal.
 48. The method of claim 47 wherein generating a second control signal to begin measuring a delay applied to the measure signal relative to the phase difference further comprises: counting several cycles of the received input clock signal to provide a stabilized input clock signal; providing a start signal to generate the second control signal to begin measuring the delay responsive to the input clock signal; and providing a stop signal to end measuring the delay responsive to the received third control signal.
 49. The method of claim 48 wherein applying the measured delay of the measure signal to the received input clock signal to provide an output clock signal synchronized with the input clock signal further comprises generating a shift selection signal to shift a unit delay of a delay line to the right or left responsive to the second control signal.
 50. The method of claim 49 wherein generating a shift selection signal to shift a unit delay of a delay line further comprises shifting an even number of unit delays in a first delay line and an odd number of unit delays in a second delay line.
 51. The method of claim 49 wherein generating a shift selection signal to shift a unit delay of a delay line further comprises shifting a unit delay of a fine delay line.
 52. The method of claim 47 further comprising generating a reset control signal to restart measuring the delay and applying the measured delay of the measure signal to the received input clock signal to provide an output clock signal synchronized with the input clock signal.
 53. A method of generating a clock signal comprising: receiving an input clock signal and outputting an output clock signal; determining a phase difference between the input clock signal and the output clock signal; initializing a delay measure mode of operation responsive to a start measure control signal; measuring a delay applied to the measure signal relative to the phase difference; stopping the delay measurement in response to a stop measure control signal; transitioning to a normal mode of operation responsive to the stop measure control signal; and applying the measured delay to the received input clock signal to generate an output clock signal having a phase locked to the phase of the input clock signal.
 54. The method of claim 53 wherein measuring a delay applied to the measure signal further comprises generating a shift selection signal to shift a delay adjustment by selecting at least one delay unit to the right or to left responsive to phase difference between the input clock signal and the output clock signal as the input clock signal is propagated through a delay line.
 55. The method of claim 54 wherein generating a shift selection signal to further comprises shifting an even number of unit delays in a first delay line and an odd number of unit delays in a second delay line.
 56. The method of claim 53 further comprising generating a reset control signal to restart the delay measure mode of operation.
 57. The method of claim 53 wherein the start measure control signal and the stop measure control signal comprise logic signals generated by at least one logic control circuit.
 58. The method of claim 57 further comprising providing a feedback signal from the output clock signal to compare to the input clock signal to determine the phase difference, and providing the feedback signal to the at least one logic control circuit to generate the start measure control signal and the stop measure control signal. 